No Receipt after 6 Weeks? How to Contact Califonia Service Cente

I filed for my parents their B2 visa extension application nearly 6 weeks ago, and the checked was cashed about 4 weeks ago. But we've NOT received the receipt. I doubt this is normal, but no one has shared their experience or knowledge after I posted a question about receipts.

I am worried that the receipt might have got lost on the way. Is there a phone number I can call CSC at? Thanks.